1. Field of the Invention
The present invention relates to a specimen analysis system which analyzes a characteristic of a specimen by injecting light into the specimen and acquiring information carried by light which propagates in and exits from the specimen.
2. Description of the Related Art
In recent years, the development of light absorption analysis (spectrometry) of a light-scattering substance such as a biological substance has been proceeding. The light applied to a light-scattering substance such as a biological substance undergoes multiple scattering and absorption in the light-scattering substance, and exits from the light-scattering substance. The behavior of light in a light-scattering substance can be expressed by an optical diffusion equation based on the optical diffusion theory, and the optical diffusion equation can be expressed as a differential equation. Therefore, it is possible to obtain a distribution of values of an optical characteristic such as an absorption coefficient, a scattering coefficient, or the like of a biological substance by measuring light which exits from the biological substance, and substituting the measured values in the optical diffusion equation.
For example, systems using the time-resolved spectroscopy and systems using the frequency-domain spectroscopy have been proposed as systems in which a distribution of optical characteristic values is obtained as above. The time-resolved spectroscopy uses ultra-short pulsed light having a width of approximately a picosecond, and the frequency-domain spectroscopy uses high-frequency modulated light.
According to the time-resolved spectroscopy, it is possible to obtain a distribution of optical characteristic values in a specimen of a light-scattering substance on the basis of the optical diffusion equation by injecting pulsed light into the specimen, and measuring the time spread (time profile) of the pulsed light which exits from the specimen after propagation through the specimen, where the time spread is caused by scattering of the pulsed light in the specimen.
According to the frequency-domain spectroscopy, it is possible to obtain a distribution of optical characteristic values in a specimen of a light-scattering substance on the basis of the optical diffusion equation by injecting high-frequency modulated light into the specimen, and measuring the intensity variation and phase delay at the modulation frequency in the light which exits from the specimen after propagation through the specimen, as disclosed by M. Vauhkonen et al., “Utilizing the radiative transfer equation in optical tomography”, OSA Biomedical Optics, pp. WF48-50, 2004.
However, in the measurement of the light which exits from a specimen after propagation in the specimen for extracting information (such as a time profile or a combination of intensity variation and phase delay at the modulation frequency) carried by the light, when the optical propagation path of the light is long (i.e., when the distance from the injection position to the measurement position of the light is great), the intensity of the detected light is low, so that it is impossible to ensure sufficiently high signal-to-noise ratio. Therefore, the reliability of analysis performed by the conventional systems using the time-resolved spectroscopy or the frequency-domain spectroscopy is low.